fairly sure its 18 1/4 but its been a bit and that was in Texas...
Its a Federal Law (National Firearms Act ("NFA"), anything under 18" you need to get the Fed Tax stamp.(civilian ownership) Just like owning a machine gun or suppressor.
The Act defines a number of categories of regulated firearms. These weapons are collectively known as NFA firearms and include the following:
Machine guns - this includes any firearm which can fire more than 1 cartridge per trigger pull. Both continuous fully-automatic fire and "burst fire" (i.e., firearms with a 3-round burst feature) are considered machine gun features. The weapon's receiver is by itself considered to be a regulated firearm. Courts have held that where a worn Firearm malfunctions in such a way as to fire multiple cartridges one or more times, this makes it a machine gun.
Short barreled rifles (SBRs) - this category includes any firearm with a buttstock and either a rifled barrel under 16" long or an overall length under 26". The overall length is measured with any folding or collapsing stocks in the extended position. The category also includes firearms which came from the factory with a buttstock that was later removed by a third party. (some smaller "trapper" type rilles are allowed though
Short barreled shotguns (SBSs) - this category is defined similarly to SBRs, but the length limit for the barrel is 18" instead of 16", and the barrel must be a smoothbore. The minimum overall length limit remains 26".
Suppressors - this includes any portable device designed to muffle or disguise the report of a portable firearm. This category does not include non-portable devices, such as sound traps used by gunsmiths in their shops which are large and usually bolted to the floor.